If you are getting a specific error message, could you tell me: What is the ? Are you using WinRAR or 7-Zip ? Knowing this will help me give you the specific fix.
Put all parts in a new, empty, isolated folder to avoid file naming conflicts during extraction. Download ifa multi rar
You do not need to extract every single part. Right-click only filename.part1.rar and select "Extract Here" (or "Extract to..."). If you are getting a specific error message,
While many programs can open RAR files, WinRAR is highly recommended, especially when dealing with multi-part archives, as it handles file reconstruction better than others. 7-Zip is a free alternative. WinRAR is highly recommended